Disclosed herein are colorants suitable for use in hydrophobic media. More specifically, disclosed herein are colorants suitable for use in applications such as phase change inks.
In general, phase change inks (sometimes referred to as “hot melt inks”) are in the solid phase at ambient temperature, but exist in the liquid phase at the elevated operating temperature of an ink jet printing device. At the jet operating temperature, droplets of liquid ink are ejected from the printing device and, when the ink droplets contact the surface of the recording substrate, either directly or via an intermediate heated transfer belt or drum, they quickly solidify to form a predetermined pattern of solidified ink drops. Phase change inks have also been used in other printing technologies, such as gravure printing, as disclosed in, for example, U.S. Pat. No. 5,496,879 and German Patent Publications DE 4205636AL and DE 4205713AL, the disclosures of each of which are totally incorporated herein by reference.
Phase change inks are desirable for ink jet printers because they remain in a solid phase at room temperature during shipping, long term storage, or the like. In addition, the problems associated with nozzle clogging as a result of ink evaporation with liquid ink jet inks are largely eliminated, thereby improving the reliability of the ink jet printing. Further, in phase change ink jet printers wherein the ink droplets are applied directly onto the final recording substrate (for example, paper, transparency material, or the like), the droplets solidify immediately upon contact with the substrate, so that migration of ink along the printing medium is prevented and dot quality is improved.
Colorants such as Pigment Red 57:1, Pigment Red 57:2, Pigment Red 57:3, and analogous laked BON-acid-type dyes (beta oxynaphthoic acid, also known as 3-hydroxy-2-naphthoic acid), as well as other —COOH and —SO3H containing dyes and their salts, are inexpensive and commercially available pigments and dyes. However, they typically are laked or metal-complexed to themselves intermolecularly as well as complexed with a small percent of abietic acid moieties (See, e.g., “British Intelligence Objectives Subcommittee Report 1661,” Publication Board No. 86139, Library of Congress, Washington, D.C., 1947, pp. 108-117, the disclosure of which is totally incorporated herein by reference. Under commercial conditions, they do not possess solubility in current commercial wax-based hot melt or phase-change ink systems. Because they are not metal-complexed to long-chain or waxy moieties, they are difficult to disperse in phase change inks, and if dispersed are not stable long-term, but settle.
Dyes are defined as materials that are soluble in a liquid on a molecular level. As such, a dye has no measurable particle size. In contrast, a pigment is not soluble in a liquid, and must be dispersed in the liquid in particulate form. Many commercially available dyes such as Pigment Red 57:1 and others are provided from the supplier with very large particle sizes and particle size distributions, and in this form are not suitable for use in phase change inks without extensive milling or grinding followed by stabilization with a dispersing agent.
A colorant that is synthesized like conventional pigments but that behaves like a dye in many ways would be highly desirable.
Accordingly, while known materials are suitable for their intended purposes, a need remains for improved colorant materials. In addition, a need remains for colorants with improved solubility or dispersibility in hydrophobic media such as waxes and resins. Further, a need remains for colorants that exhibit stable solubility or dispersibility over time in phase change ink media. Additionally, a need remains for colorants that can be obtained chemically as submicron-sized particles with no need for grinding, dispersing, milling, homogenizing, or similar mechanical methods for reducing particle size. There is also a need for colorants that have a measurable particle size in liquids but that require no milling to reduce the particle size after they are synthesized. In addition, there is a need for colorants that exhibit both pigment-like and dye-like behavior in that they have measurable particle size but in many instances need not be dispersed in the liquid by physical or chemical means.